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Author Topic: Lab Color for B/W - Why does it work so well?  (Read 11481 times)
papa v2.0
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« Reply #20 on: February 19, 2010, 06:25:00 AM »
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If you want to work in LAB its fine. Once you have removed AB channels or de-saturated the image (Or what ever way suits you). Keep the image in LAB space and print direct from the LAB file. (image is tagged with LAB profile, either convert to printer profile to preview and print or print direct, using relative colormetric)

Reason is most  profiles  use LAB as the profile connection space. So let the printer profile do the LAB conversion to the printer space and see what you get. Dont go RGB to LAB to RGB and print.

Its the way i would go and it will save you s. This is the method we use to send test targets to the printer to evaluate printer profile performance.

Try printing off a stepwedge.
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« Reply #21 on: February 19, 2010, 07:45:15 AM »
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Quote from: Mike Arst
I was wrong earlier when I described a B&W conversion technique as being provided by Caponigro. This is the one provided by Caponigro: http://www.adobe.com/digitalimag/ps_pro_primers.html -- see the link "Black and White Conversion Tutorial" (PDF file).

The technique involves copying and pasting the individual R, G and B channels into the main document, creating a new layer each time you paste. I've tried it a number of times in Photoshop CS2 and haven't been able to get it to work. With each "paste," the new layer that appears in the document contains the data only for the very first channel that was copied. That is, if I first paste the Red channel, at the point when I've copied and pasted the Blue channel, it's still only the red-channel data that appears in the new layer. The same happens when I copy and paste the green-channel data: again I get only the red-channel data in the new layer.

I've been following the instructions slowly and meticulously -- or so I thought -- but clearly I'm doing something wrong. Can anyone reading this message think what it might be? Or, is it possible there's something missing from those instructions?

The web page contains a Photoshop action that's said to automate the sequence of events described in the tutorial. That isn't quite correct. The action does considerably more than is described in the tutorial. When it is run, the channels when pasted into the main document DO look the way I would expect them to look.

(I realize that Silver Efex Pro is a mere $200. Despite the bargain pricing, I would nevertheless like to find some other workable approach, if possible. To date, Convert to B&W Pro having been discontinued, I have not yet found a decent plug-in for this purpose...)

Have you exhausted the potential of the B&W panel in Lightroom/ACR and the B&W Adjustment Layer in Photoshop? You may well find that by properly using an up-to-date version of Photoshop you simply don't need plug-ins and convoluted workflows to get great results. There's far too much hype all over the place about making B&W conversions, when most often the easiest and most direct approaches are all that one needs.
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Mark D Segal (formerly MarkDS)
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« Reply #22 on: February 19, 2010, 07:49:25 AM »
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Quote from: papa v2.0
Reason is most  profiles  use LAB as the profile connection space. So let the printer profile do the LAB conversion to the printer space and see what you get. Dont go RGB to LAB to RGB and print.

Moat inkjet printers are RGB devices and their drivers are written to expect RGB file numbers in RGB colour space. The printer firmware converts these numbers under the hood to CMYK for printing, because the inks are CMYK inks. I don't recommend interfering with that workflow in general and in particular for the sake of using the "L" channel in L*a*b* to make a B&W print.
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Mark D Segal (formerly MarkDS)
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« Reply #23 on: February 19, 2010, 07:53:51 AM »
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Quote from: Mike Arst
I was wrong earlier when I described a B&W conversion technique as being provided by Caponigro. This is the one provided by Caponigro: http://www.adobe.com/digitalimag/ps_pro_primers.html -- see the link "Black and White Conversion Tutorial" (PDF file).

The technique involves copying and pasting the individual R, G and B channels into the main document, creating a new layer each time you paste. I've tried it a number of times in Photoshop CS2 and haven't been able to get it to work. With each "paste," the new layer that appears in the document contains the data only for the very first channel that was copied. That is, if I first paste the Red channel, at the point when I've copied and pasted the Blue channel, it's still only the red-channel data that appears in the new layer. The same happens when I copy and paste the green-channel data: again I get only the red-channel data in the new layer.

I've been following the instructions slowly and meticulously -- or so I thought -- but clearly I'm doing something wrong. Can anyone reading this message think what it might be? Or, is it possible there's something missing from those instructions?

The web page contains a Photoshop action that's said to automate the sequence of events described in the tutorial. That isn't quite correct. The action does considerably more than is described in the tutorial. When it is run, the channels when pasted into the main document DO look the way I would expect them to look.

(I realize that Silver Efex Pro is a mere $200. Despite the bargain pricing, I would nevertheless like to find some other workable approach, if possible. To date, Convert to B&W Pro having been discontinued, I have not yet found a decent plug-in for this purpose...)

Do you use LightRoom? Its black and white conversion tools are really quite good.
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« Reply #24 on: February 19, 2010, 08:06:22 AM »
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Quote from: PeterAit
Do you use LightRoom? Its black and white conversion tools are really quite good.

Exactly - and the same tools are in recent versions of Camera Raw and Photoshop.
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Mark D Segal (formerly MarkDS)
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« Reply #25 on: February 19, 2010, 08:27:31 AM »
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All of this has been most interesting, and I do thank everyone for taking the time and trouble to contribute to this debate. Now, you are welcome to correct me if I am wrong, but essentially there seem to be the following options if one wishes to use a digital camera or MF digital back and produce not colour images, but B/W prints.

1) Convert to Grayscale from RGB in Image>Mode>Grayscale. This actually is not a straight desaturation, but is more like 30% R, 60% G, and 10% B (according to the info I have found).

2) Desaturate the RGB image in Image>Adjustments>Desaturate. Or desaturate in ACR or other RAW converter using the saturation slider set to zero. This should produce a file which has exactly equal quantities of RGB.

Both of these strategies produce a file which, to my eyes, is rather flat and lifeless. Others may disagree.

3) Use Channel Mixer in PS to mix varying proportions of the R, G and B channels to a monochrome image. This can be understood in two ways - either as attempting to replicate the effect of using coloured filters on the lens as we used to do when shooting film, or attempting to replicate the non-linear response of B/W film stock, or indeed both at once. Most, if not all, of the other plugins, things in PS or CS4 or whatever are just variations or more sophisticated versions of this basic theme. (Some of them are actually quite good - the FP4 profile in CBW Pro is pretty believable. If you like FP4, of course.)

4) Convert the RGB image to Lab Color in Image>Mode>Lab Color and then extract the L channel. CIE Lab seems to produce something genuinely different which is easily differentiated from the other separation methods when you make a print on good-quality stock. Whether you like it better or not is a matter of taste.

Of course, all the above are just the beginning of the process for a fine quality B/W print. Thereafter must follow all the usual adjustments for levels, local contrast, dodging and burning just as we used to do in the darkroom. Essentially, what I am looking for is the perfect negative as my starting point, except that now it is a positive. Following Ansel's practice, it should be correctly exposed, retain the crucial highlight detail, have excellent tonal separation in the all-important mid-range, and have normal contrast. As he so rightly pointed out, we can fairly easily add contrast, but it is much more difficult to reduce it if the crucial upper-mids have been lost.

John
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« Reply #26 on: February 19, 2010, 09:03:38 AM »
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Quote from: John R Smith
All of this has been most interesting, and I do thank everyone for taking the time and trouble to contribute to this debate. Now, you are welcome to correct me if I am wrong, but essentially there seem to be the following options if one wishes to use a digital camera or MF digital back and produce not colour images, but B/W prints.

1) Convert to Grayscale from RGB in Image>Mode>Grayscale. This actually is not a straight desaturation, but is more like 30% R, 60% G, and 10% B (according to the info I have found).

2) Desaturate the RGB image in Image>Adjustments>Desaturate. Or desaturate in ACR or other RAW converter using the saturation slider set to zero. This should produce a file which has exactly equal quantities of RGB.

Both of these strategies produce a file which, to my eyes, is rather flat and lifeless. Others may disagree.

3) Use Channel Mixer in PS to mix varying proportions of the R, G and B channels to a monochrome image. This can be understood in two ways - either as attempting to replicate the effect of using coloured filters on the lens as we used to do when shooting film, or attempting to replicate the non-linear response of B/W film stock, or indeed both at once. Most, if not all, of the other plugins, things in PS or CS4 or whatever are just variations or more sophisticated versions of this basic theme. (Some of them are actually quite good - the FP4 profile in CBW Pro is pretty believable. If you like FP4, of course.)

4) Convert the RGB image to Lab Color in Image>Mode>Lab Color and then extract the L channel. CIE Lab seems to produce something genuinely different which is easily differentiated from the other separation methods when you make a print on good-quality stock. Whether you like it better or not is a matter of taste.

Of course, all the above are just the beginning of the process for a fine quality B/W print. Thereafter must follow all the usual adjustments for levels, local contrast, dodging and burning just as we used to do in the darkroom. Essentially, what I am looking for is the perfect negative as my starting point, except that now it is a positive. Following Ansel's practice, it should be correctly exposed, retain the crucial highlight detail, have excellent tonal separation in the all-important mid-range, and have normal contrast. As he so rightly pointed out, we can fairly easily add contrast, but it is much more difficult to reduce it if the crucial upper-mids have been lost.

John

Approaches (1) and (2) either destroy a lot of information you should keep or they produce sub-optimal results or both - you can read why elsewhere, but your observation that they're no good is correct; approaches (3) and (4) are more convoluted than necessary to make a very high quality B&W rendition, and if you don't have the application engineering background to know what is "just variations" of what, it's probably not productive to speculate - in any case it's only the results that matter. Maybe it would be a productive suggestion to stop ignoring the more straightforward and highly effective advice being offered to you here, for example in posts 7, 23, 24 and 25 - or is it that you prefer convoluted, dated workflows in order to avoid up-grading your software, or are you using file formats which the latest Capture-1, Lightroom and Photoshop versions can't handle (unlikely)? I think at the very least you should give the newer, more straightforward tools a whirl (free trial demos) before coming to conclusions what's been discussed here. Or have you done that already and found it not to your liking?
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Mark D Segal (formerly MarkDS)
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« Reply #27 on: February 19, 2010, 09:22:52 AM »
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Quote from: Mark D Segal
Maybe it would be a productive suggestion to stop ignoring the more straightforward and highly effective advice being offered to you here, for example in posts 7, 23, 24 and 25 - or is it that you prefer convoluted, dated workflows in order to avoid up-grading your software, or are you using file formats which the latest Capture-1, Lightroom and Photoshop versions can't handle (unlikely)? I think at the very least you should give the newer, more straightforward tools a whirl (free trial demos) before coming to conclusions what's been discussed here. Or have you done that already and found it not to your liking?

Well, Mark, all I was doing above was listing the choices which are available to me. And I did say that you were welcome to correct me if I am wrong. I am not trying to ignore anything, but to be frank, extracting the L channel is hardly "convoluted", really, is it? Two or three mouse clicks, actually. My question at the beginning of this thread was not which B/W strategy is best, simply why (technically) does the L channel look different from a straight desaturation. That was all. I still haven't really had an answer to that.

John
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« Reply #28 on: February 19, 2010, 09:45:27 AM »
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Quote from: John R Smith
Well, Mark, all I was doing above was listing the choices which are available to me. And I did say that you were welcome to correct me if I am wrong. I am not trying to ignore anything, but to be frank, extracting the L channel is hardly "convoluted", really, is it? Two or three mouse clicks, actually. My question at the beginning of this thread was not which B/W strategy is best, simply why (technically) does the L channel look different from a straight desaturation. That was all. I still haven't really had an answer to that.

John
John, I hear you, but converting to L*a*b* and back is destructive (this is a huge old debate I'm not trying to revive and won't respond to - you can find it all on the internet - but it's there and should not be ignored). It is only a few mouse clicks, but it's a few mouse clicks more than you need with more up-to-date software that gives you access to better tools; and unless you keep the original file, make a copy, do the L*a*b* thing on the copy, convert it back to RGB, and then copy this reconverted B&W to RGB onto the original RGB file as a layer, it is non-reversible relative to a workflow which simply keeps your B&W work on a separate Adjustment Layer natively if a rendered PSD or TIFF, or as metadata in a raw file which is easliy reversed or amended. So yes, considering all this, using L*a*b* is convoluted and is inferior from a workflow perspective. I'm not arguing that it can't deliver good results - I am arguing that it's needless and quite likely sub-optimal.

Now, as to why the "L" channel looks different from a deaturated version of all three RGB channels, I don't know. I'm not a software engineer and I don't know the colour space math which would explain this. Superficially though, one would expect them to be different because the colour spaces are constructed differently. I can conjure up in my mind the processes which may give rise to those differences, but that isn't a useful substitute for really knowing the answer. You are right to ask for forum members to answer your original question - if they know how   - that's fair ball as we do tend to get carried-away from the OP's original questions in these discussions.
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Mark D Segal (formerly MarkDS)
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« Reply #29 on: February 19, 2010, 10:04:38 AM »
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Quote from: John R Smith
.. why (technically) does the L channel look different from a straight desaturation.

One way to obtain typical straight desaturation is to take linear R,G,B and multiply them by some weights (derived with some reference white point in mind), which gives you a single number, the relative luminance. Now desatruation is obtained by replicating this single number in R, G, and B. You can convert it to non-linear data by applying a gamma function, where a typical value would be 2.2, i.e., inverse gamma of 1/2.2=0.45. However, for L in the Lab, you would take that relative luminance value, and using the relative luminance value of reference white you would raise it to the inverse gamma of 0.33; the actual process is slightly more complicated with some multiplicative and subtractive constants. So these inverse gammas of 0.45 (or what ever value was chosen by your software program) and the 0.33 in Lab, (together with those multiplicative/subtractive constants) would make the difference.

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« Reply #30 on: February 19, 2010, 12:11:01 PM »
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Quote from: Mark D Segal
Moat inkjet printers are RGB devices and their drivers are written to expect RGB file numbers in RGB colour space. The printer firmware converts these numbers under the hood to CMYK for printing, because the inks are CMYK inks. I don't recommend interfering with that workflow in general and in particular for the sake of using the "L" channel in L*a*b* to make a B&W print.


Assuming you are using a profile based work flow the image in RGB say sRGB for arguments sake is tagged with a sRGB profile. to print to the printer the file is converted to printer space via the PCS. The PCS is LAB, (well is for my epson inkjet). so the data will go from sRGB to LAB and then EPSON RGB SPACE  and then internally to cmyk probably gamut mapped from LAB space anyway.

so the input to epson RGB space is via LAB not RGB to RGB.
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« Reply #31 on: February 19, 2010, 12:42:28 PM »
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Quote from: papa v2.0
Assuming you are using a profile based work flow the image in RGB say sRGB for arguments sake is tagged with a sRGB profile. to print to the printer the file is converted to printer space via the PCS. The PCS is LAB, (well is for my epson inkjet). so the data will go from sRGB to LAB and then EPSON RGB SPACE  and then internally to cmyk probably gamut mapped from LAB space anyway.

so the input to epson RGB space is via LAB not RGB to RGB.

However the data gets handled in the CMS and PCS, the print workflow expects an RGB starting point, and the printer profiles we use in Photoshop are also based on reading RGB input data, regardless of whether the CMS and PCS uses CIE L*a*b* or CIE XYZ as the conversion platform. It is true that you can convert your RGB file to L*a*b* which means flattening your image thereby destroying all further access to your adjustment layers (another reason not to use L*a*b* unless you have a duplicate image) and send it to print, and it will print. I have done this and gotten good results, but I don't know (and therefore don't recommend) how reliable this would be accross all images I would print. If you have thoroughly tested and are unequivocally certain that you can begin the print process from Photoshop with L*a*b* data and the resulting print quality will be systematically identical to starting with RGB data in the usual and most recommended way, good to know.
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Mark D Segal (formerly MarkDS)
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« Reply #32 on: February 19, 2010, 11:51:37 PM »
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Quote from: John R Smith
My question at the beginning of this thread was not which B/W strategy is best, simply why (technically) does the L channel look different from a straight desaturation. That was all. I still haven't really had an answer to that.
The difference is that the L channel in Lab is based on the strict CIELAB definition of perceptual lightness and its calculation accounts for the color space and gamma of an image. The result is that the same gray tone will be assigned to a color no matter what the color space. See www.brucelindbloom.com, "Math", for the RGB-to-XYZ and XYZ-to-Lab equations.

The RGB methods use various approximations that seem to ignore the color space and gamma of the image. The result is that the gray tone assigned to a color will vary depending on the color space the image happens to be in. In many cases there is only a rough relationship to perceived lightness. As an example, the Grayscale command appears to involve mixing the RGB channels as: .30*R + .59*G + .11*B. The same ratio is used even though the RGB values for non-neutral colors are different in different color spaces. The mixing is done without regard to whether the RGB values are gamma compressed or not (in contrast, the mixing in Lab is done in the linear XYZ space.)

The Desaturate command appears to use yet another method, resulting in different and varying tones.

Bottom line, the L channel looks different because it's calculated differently.
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« Reply #33 on: February 20, 2010, 02:54:24 AM »
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Quote from: Mark D Segal
Have you exhausted the potential of the B&W panel in Lightroom/ACR and the B&W Adjustment Layer in Photoshop? You may well find that by properly using an up-to-date version of Photoshop you simply don't need plug-ins and convoluted workflows to get great results. There's far too much hype all over the place about making B&W conversions, when most often the easiest and most direct approaches are all that one needs.
I definitely have not exhausted these approaches. In fact I'm behind the curve in terms of Photoshop versions, not having felt any burning need to upgrade from CS2 to CS4 -- until, that is, I began to hear some renewed praise for recent versions of ACR (which I had abandoned some time back as not producing especially good results compared with other RAW converters). I hope it has improved in its rendering of fine image detail. This was a clear superiority that I noticed in Capture One, RawShooter Premium, and even the new version of Bibble. All of these beat ACR in terms of fine image detail. I hope ACR has improved in that regard (Lightroom's 1.x versions, at least, didn't impress me fine-image-detail-wise).

Clearly I'd best give the ACR solution a shot (meaning, give CS4 a shot; the ACR version I have doesn't support my camera's format). I have never been fond of Lightroom. I hear, though, that ACR's development "engine's" and LR's are now "identical" (with ACR not having the targeted-adjustments feature). Could this mean that ACR is also now written in LUA? It's hard to imagine how they could be "identical" otherwise. At any rate, having purchased one set of plug-ins that contains an ostensibly decent b&w converter but which turns out to have some fairly serious bugs -- what a disappointment -- I guess I should stop balking at upgrading to CS4...and give it and ACR's b&w solutions a try. Thanks for your feedback about it.
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« Reply #34 on: February 20, 2010, 02:59:05 AM »
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Quote from: PeterAit
Do you use LightRoom? Its black and white conversion tools are really quite good.
I haven't used the most recent versions of Lightroom. I became extremely un-fond of its more bizarre UI "features" and for the most part stopped using it. This of course has nothing to do with recent versions' b&w capabilities, and I will take your word about them. If ACR's b&w capability is comparable, I think I'll start there. Being forced into a D.A.M. solution, especially LR's kind of unpleasant implementation, doesn't appeal to me (unless they have made some major progress in the UI dept.). But b&w does appeal to me. :-) Thanks for the feedback about this.
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« Reply #35 on: February 20, 2010, 03:05:33 AM »
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Quote from: Mark D Segal
Exactly - and the same tools are in recent versions of Camera Raw and Photoshop.

When you use the convert mode in Camera raw and you import to Photoshop are you changing the mode from grayscale to RGB?
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« Reply #36 on: February 20, 2010, 08:11:08 AM »
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When you use the convert mode in Camera raw and you import to Photoshop are you changing the mode from grayscale to RGB?
It exports to Photoshop as a three channel RGB image. Try it. You'll see the three channels. Turning off one channel at a time, you'll get the opponent colour of the channel you turned off. But when all three channels are active, you see greyscale. However, you are never stuck with that - in case you want to go back to colour, you can revert to the raw file and flip back to Color and re-render it to Photoshop, or you can have the raw version embedded in Photoshop as a Smart Object, which makes it that much more convenient to flip around between various raw states and Photoshop editing. Note however you can't do pixel-editing on a Smart Object.
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« Reply #37 on: February 20, 2010, 08:25:08 AM »
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Quote from: Mike Arst
I haven't used the most recent versions of Lightroom. I became extremely un-fond of its more bizarre UI "features" and for the most part stopped using it. This of course has nothing to do with recent versions' b&w capabilities, and I will take your word about them. If ACR's b&w capability is comparable, I think I'll start there. Being forced into a D.A.M. solution, especially LR's kind of unpleasant implementation, doesn't appeal to me (unless they have made some major progress in the UI dept.). But b&w does appeal to me. :-) Thanks for the feedback about this.

Mike, I own no stock in Adobe and I have no vested interest in pushing their software. Let me just put it to you this way: a very large number of highly successful photographers are using Lightroom to achieve a high proportion of their total workflow and they are making a fine living from it. If it produced crappy conversions and sub-optimal output they would be using something else. I have processed thousands of images in Lightroom, and the detail I can see in print, on paper, is fine - and it has passed muster with peer reviewers who have sharp eyesight and decades of experience looking at prints. So all these opinions I keep reading about the inferior quality of Adobe raw converters leaves me thoroughly unimpressed. You need to distinguish between pixel-peeping as a hobby in its own right, and real world results which people see on paper and judge with their wallets. Yes, anything can always be better, but that goes for everything. The ACR and LR processing engines are the same; the GUI differs. I like Lightroom's GUI better, but that's not a fundamental. As for the file management, again, a great many professional photographers buy this program especially because of its image management capability - in the hands of people who take the time to learn how to use it. This is a major strength of the program. Once you get accustomed to the fact that it ISN'T a file browser, and you begin to "wear it's shoes" so to speak, you can use and appreciate what it can do. Up to now, it has always been worthwhile up-dating Photoshop and Lightroom to take advantage of the newer features. There is and always will be place for high quality third party plugins to perform discrete operations, but they complement rather than replace the advantage of up-grading the base software to which they relate.
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« Reply #38 on: February 20, 2010, 09:25:23 AM »
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Quote from: crames
the Grayscale command appears to involve mixing the RGB channels as: .30*R + .59*G + .11*B. The same ratio is used even though the RGB values for non-neutral colors are different in different color spaces. The mixing is done without regard to whether the RGB values are gamma compressed or not (in contrast, the mixing in Lab is done in the linear XYZ space.)

You are right about using incorrect coefficients, and IIRC, even some SMPTE documents have used "traditional" coefficients in lieu of "correct" coefficients. But, if one does not know the right primaries for a given RGB data, then wouldn't even RGB->XYZ conversion be incorrect before going XYZ->Lab?
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« Reply #39 on: February 20, 2010, 09:37:17 AM »
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Quote from: joofa
You are right about using incorrect coefficients, and IIRC, even some SMPTE documents have used "traditional" coefficients in lieu of "correct" coefficients. But, if one does not know the right primaries for a given RGB data, then wouldn't even RGB->XYZ conversion be incorrect before going XYZ->Lab?
Yes, but all the information you need to convert from RGB to XYZ is contained in the color profile, or working color space definition. That is what is used when converting from RGB to Lab in PS.
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Cliff
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